1. The Field of the Invention
This invention relates to locking devices and more particularly to that class of locking mechanism which, when forced or when tampered with, in an unauthorized fashion, presents a broken mechanism which is substantially tamperproof.
2. Description of the Prior Art
The prior art abound with locking devices, of the pick resistant or tamperproof variety. U.S. Pat. No. 4,056,955 issued Nov. 8, 1977 to D. W. Glass teaches an automobile ignition lock which includes a plug having a plurality of tumblers. The plug is rotatably mounted in a cylinder and includes a slot for receipt of a tumbler actuating key. A key bolt is positioned at the inner end of the key slot and is moveable in response to the insertion of the key in the key slot from a plug locking to a plug unlocking position. Excessive longitudinal transfer or movement of the key bolt relocks the plug and cylinder and prevents relative rotation. Thus, a key with appropriate bitting and of appropriate length must be used to effectuate unlocking the plug from the cylinder. As an additional security feature of the lock, the plug is designed with the circumferential slot that fractures and separates the plug into two separate portions when an attempt to forceably rotate the plug relative to the cylinder. Unfortunately, the glass apparatus may be successfully picked by two successive plug removeable operations. Thus, the lock can be removed by first exerting a longitudinal force to that portion of the plug which is located adjacent the key slot. After removing same, exerting a successive longitudinal force upon the remaining portion of the plug exposes the non-circular portion of the apparatus which is coupled to the locking mechanism of the lock.
U.S. Pat. No. 3,863,476 issued on Feb. 4, 1975 to G. P. Patriquin discloses a cylinder lock of the plug rotatably mounted in a housing and including key way for receiving a key. A cam actuator operates the lock controlling cam and is rotatably mounted in the housing behind the plug and receives the end of the key as it passes through the plug. One or more remaining restraining wafers interact with the proper key and releasably hold the cam actuator in a lock position with respect to the housing. Restraining wafers in the plug maintains it in a preselected position with respect to the cam actuator and the housing. A proper key is received by both the plug and cam actuator and releases both for rotational motion to unlock the device and supplies torque to both therefor. Attempting to pick the subject lock in the normal manner only releases the plug from the cam actuator and thus the lock remains locked. The spline arrangement is such that the cam actuator cannot be picked without first picking the plug, thus, rendering the lock time consuming to pick even for one familiar with its construction. Auxiliary splines are provided and the wafers can define contoured edges for interacting the plurality of splines simultaneously. However, once the plug portion has been released the cam actuator remains and may be operated upon the insertion of a screwdriver in the housing through the passageway formerly retaining the plug. Deficient force, if employed, may override the restraining forces provided by the restraining wafers, thus providing for easy rotation of the lock controlling cam.
U.S. Pat. No. 4,008,588 issued on Feb. 22, 1977 to H. C. Miller et al. discloses a key operated rotary plug cylinder lock having rotary disc tumblers with a concentric cylindrical boss on one face of each disc tumbler projecting actually into internesting rotation into a complemental cylindrical well or recessed in the confronting face of the next adjacent disc tumbler. A lock out pin mechanism to lock the plug against rotation when excessive rearward stress is exerted thereon and a frangible lost motion coupling between the tumblers and the shell portion of the plug breakable upon excess torquing forces are also provided. The apparatus is only effective upon the use of a cylindrically shaped key and can be successfully picked upon crude alignment attempts of the rotary disc tumblers, sufficient to allow an L-shaped instrument passing therethrough. Upon pulling out each or all of the disc-shaped elements, the shell portion of the plug is accessible for rotation by the simple insertion of a large tool, such as the broad face of a screwdriver.